Diminishing fuel reserves and the effects of climate change have highlighted the need to make the best use of all available energy sources. Solar energy is probably the most significant renewable energy source—it also has the advantage of minimal environmental impact. The development of technology for exploiting solar energy is very important for future energy production.
Solar energy can be utilized in a number of ways but for the production of 5-500 kW electrical power in dry sunny regions, where water is scarce, there are two leading possibilities: concentrated photo-voltaic (CPV) systems; and solar heated Stirling engines driving electrical generators.
What is needed for both are low cost solar concentrators that can produce high concentrations of at least 500 and preferably up to values in excess of 1000. For photo-voltaics, high efficiency Photo-Voltaic materials are expensive and high concentrations allow them to be used more cost effectively. For Stirling engines, high efficiency requires high heater temperatures and this can only be achieved with high levels of concentration.
Currently solar concentrators used to provide these high levels of concentration use designs that require reflective surfaces that are fully three dimensional. For example, in US2006266408 (Solfocus) a two stage concentrator is described that can provide high concentrations for Photo-Voltaic devices. The two mirrors are made out of glass and are silvered to give the required reflective coatings. The mirror profiles have curvature in two directions.
For Stirling engine applications, designs have centered on single stage parabolic reflectors that produce a point focus. Spherical reflectors have also been used—for high values of f/D (focal length/dish diameter) an approximate point focus is produced with a high enough concentration factor to be used in these applications. Alternatively, arrays of spherical reflectors have been used that approximate a parabolic reflector. Again the various reflector profiles have curvature in two directions.
Relatively expensive manufacturing techniques and materials are generally required to form the reflector profiles having curvature in two directions to the required accuracy. In addition, handling and transport of the reflective components from the factory to the installation site can be difficult, due to their bulk and potential fragility.
As well as the design of the basic concentrator system, other aspects of a complete solar generator system that need to be addressed concern the design of the cavity or window type receivers for Solar Stirling generators, aspects such as the angle of incidence for Photo-Voltaic generators, and more generally non-imaging “secondary” concentrators for both.
The principles used for the design of cavity receivers and non-imaging concentrators, such as Winston cones, are well established and known to those skilled in the art. See, for example, the following: 1) Pitz-Paal Robert. High Temperature Solar Concentrators. in Solar Energy Conversion and Photoenergy Systems in Encyclopedia of Life Support Systems. Eolss Publishers, Oxford UK 2007; and 2) Roland Winston, Juan C. Minano, Pablo Benitez, (with contributions by Narkis Shatz and John C. Bortz), Nonimaging Optics, Academic Press, 2004. (ISBN 0-12-759751-4). Also commercial software exists that greatly facilitates the design of these components. See, for example, ZEMAX, Optima Research Ltd, 8 Riverside Business Park, Stoney Common Road, Stansted, CM24 8PL, United Kingdom e.g. ZEMAX (ref 4). These tools are particularly useful for more detailed requirements such as attaining uniform temperature of the heater assembly or uniform illumination of the photo-voltaic cell.